sensors Article
Construction of Multiple Switchable Sensors and Logic Gates Based on Carboxylated Multi-Walled Carbon Nanotubes/Poly(N,N-Diethylacrylamide) Xuemei Wu 1,† , Xiaoqing Bai 2,† , Yang Ma 1,† , Jie Wei 1 , Juan Peng 3 , Keren Shi 4 and Huiqin Yao 2, * 1 2 3 4
* †
College of Pharmacy, Ningxia Medical University, Yinchuan 750004, China;
[email protected] (X.W.);
[email protected] (Y.M.);
[email protected] (J.W.) School of Basic Medical Sciences, Ningxia Medical University, Yinchuan 750004, China;
[email protected] School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, China;
[email protected] State Key Laboratory of High-Efficiency Coal Utilization and Green Chemical Engineering, Ningxia University, Yinchuan 750021, China;
[email protected] Correspondence:
[email protected]; Tel.: +86-951-698-0110 These authors contributed equally to this work.
Received: 3 September 2018; Accepted: 4 October 2018; Published: 8 October 2018
Abstract: In this work, binary hydrogel films based on carboxylated multi-walled carbon nanotubes/poly(N,N-diethylacrylamide) (c-MWCNTs/PDEA) were successfully polymerized and assembled on a glassy carbon (GC) electrode surface. The electroactive drug probes matrine and sophoridine in solution showed reversible thermal-, salt-, methanol- and pH-responsive switchable cyclic voltammetric (CV) behaviors at the film electrodes. The control experiments showed that the pH-responsive property of the system could be ascribed to the drug components of the solutions, whereas the thermal-, salt- and methanol-sensitive behaviors were attributed to the PDEA constituent of the films. The CV signals particularly, of matrine and sophoridine were significantly amplified by the electrocatalysis of c-MWCNTs in the films at 1.02 V and 0.91 V, respectively. Moreover, the addition of esterase, urease, ethyl butyrate, and urea to the solution also changed the pH of the system, and produced similar CV peaks as with dilution by HCl or NaOH. Based on these experiments, a 6-input/5-output logic gate system and 2-to-1 encoder were successfully constructed. The present system may lead to the development of novel types of molecular computing systems. Keywords: electrocatalysis; encoder; logic gate; matrine; sophoridine
1. Introduction Molecular logic gates can carry out different computations at the molecular level, and the working mechanism or principle is similar to silicon processors [1–4]. Molecular logic gates, a type of logic gates system, can use biological materials or biomolecules such as enzymes, DNA, or proteins to establish basic or -simple molecular devices and equipment [3–7]. For example, Liu’s group successfully established 4-input/7-output biomolecular logic gates, and provided a new method to construct complex biomolecular logic gate system based on bio-electrocatalysis of natural DNA [8]. An adjustable electrocatalysis was constructed on the basis of multiple responsive interface poly (N-isopropylacrylamide co-N,N’-dimethylaminoethylmethacrylate) (P(NiPAAm-DMEM) films while the immobilization of glucose oxidase (GOD) and horseradish peroxidase (HRP) on the surface of pyrolytic graphite (PG) electrodes was also used to construct logic gates by Liu and coworkers [9]. Sensors 2018, 18, 3358; doi:10.3390/s18103358
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Because of their huge potential in different areas, including disease diagnosis, sensing, and drug release [10–12], the molecular logic gate system has attracted ever increasing attention. In previous reports, the widely used probe compounds were ferrocene and its derivatives including ferrocene carboxylic acid, ferrocenylmethanol, ferrocene dicarboxylate, potassium ferrocyanide, and hydroquinone [1,13–16]. For example, Hu’s group [17] studied the switchable behaviors of poly(N-isopropylacrylamide) (PNIPAA)-GOD film using ferrocene carboxylic acid as an electroactive probe. The permeability of the reduced graphene oxide/poly(N-isopropylacrylamide) (rGO/PNIPAA) dual structure film was investigated by using ferrocene dicarboxylate as a molecular probe, and established a 4-input enabled OR (EnOR) logic gate system [18]. However, drug molecules are becoming increasingly popular probes in the field of electrochemistry and medical research, because some drug molecules with electroactivity can be used to investigate the properties of film systems in vitro or used as target drugs to treat diseases in vivo. Matrine and sophoridine are quinolizidine alkaloids isolated from traditional Chinese medicine such as Sophora flavescens, Sophora subprostrata, or Sophora alopecuroides, which are tetracyclic quinolizine compounds with a molecular skeleton and can be regarded as a combination of two quinolizine rings [19,20]. In recent years, it has been discovered that the alkaloids of S. flavescens exhibit anti-arrhythmic, anti-inflammatory, anti-fibrosis, anti-tumor, and numerous other significant pharmacological activities, and various formulations have been widely used in clinical practice [21–24]. Therefore, in the present work, we used matrine and sophoridine as target drug molecule probes to study the switching properties of the binary carboxylated multi-walled carbon nanotubes/poly(N,N-diethylacrylamide) (c-MWCNTs/PDEA) film system. The electrochemical signals of matrine and sophoridine were amplified by means of electrocatalysis, and the 6-input/5-output logic gate systems were constructed. Currently, the logic gate system has become a research hotspot [3] and its applications for clinical drugs are expected in future clinical diagnosis and treatment research. PDEA hydrogel films were synthesized on glassy carbon (GC) electrodes by cross-linking radical polymerization in this system. PDEA is a widely investigated sensitive polymer, its temperature and salt sensitivity have been reported in previous literature [25–27]. However, its methanol sensitivity, especially combined with its temperature and sulfate concentration sensitive on–off property has never been reported by the cyclic voltammetry (CV) method. This inspired us to increase the complexity of the logical gate system by increasing the number of outside stimuli. In the present work, the electroactive drug molecules, matrine and sophoridine, showed CV on–off behaviors at c-MWCNTs/PDEA-film electrodes upon external stimuli of pH, temperature, sulfate concentration, and methanol proportion. A logic gate system and 2-to-1 encoder were further constructed. This may open up the possibility of applying electroactive drug molecular probes in electrochemical biosensors or other devices based on multiple responsive electrocatalysis. 2. Experimental Section 2.1. Reagents N,N’-methylenethyl butyrateisacrylamide (BIS), urease (E.C. 3.5.1.5, type III, 39, 290 units g−1 ), esterase (17,000 units g−1 ), and N,N,N’,N’-tetramethylenediamine (TEMED) were obtained from Sigma Aldrich (Beijing, China) and used without further purification. Matrine and sophoridine were purchased from Ningxia Zijinghua Pharmaceutical Co., Ltd. (Yinchuan, China). Multiwalled carbon nanotubes (MWCNTs) were obtained from Shenzhen Nanotech Port Co. Ltd. (Shenzhen, China). N,N-diethylacrylamide (DEA) was purchased from TCI (Shanghai, China). Sodium persulfate (Na2 S2 O8 ) was purchased from Aladdin (Shanghai, China). Sodium sulfate (Na2 SO4 ), sodium nitrate (NaNO3 ), potassium nitrate (KNO3 ), sodium chloride (NaCl), and magnesium sulfate (MgSO4 ) were purchased from Tianjin Haiguang Chemical Plant (Tianjin, China). All of the other reagents were of analytical grade. All solutions contained 0.15 M NaCl, and the pH was adjusted using dilute HCl or NaOH. All solutions were prepared with water purified twice by ion exchange and distillation.
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2.2. Preparation of Poly(N,N-Diethylacrylamide) Hydrogel Films The typical pre-gel solution containing 0.4 mg mL−1 Na2 S2 O8 initiator, 1.5 mg mL−1 BIS cross-linker, 0.46 mg mL−1 TEMED accelerator, and 0.5 M DEA monomer, was freshly prepared every time and the air removed with N2 before being cast. After about 8 min, the PDEA gel was formed on the surface of the glassy carbon (GC) electrode. During the whole polymerization process, the N2 atmosphere was maintained in the bottle. The formed PDEA-film electrode was then immersed in water for 20 min to remove the unreacted reagents. 2.3. Apparatus and Procedures A CHI 660E electrochemical workstation (CH Instruments, Beijing, China) was used for electrochemical measurements using a typical three-electrode cell with a platinum flake as the counter electrode, a saturated calomel electrode as the reference, and the GC electrode with films as the working electrode. The pH measurements were performed using a PHS-3C pH meter (Shanghai Precision & Scientific Instruments, Shanghai, China). The temperature of solutions in the cell was controlled by a DK-S14 thermostatic bath (Shanghai Jinghong Experimental Equipment Co. Ltd., Shanghai, China) with the precision of ±0.5 ◦ C. The Fourier-transform infrared (FTIR) spectra were recorded at a resolution of 4 cm−1 with a Spectrum Two FTIR spectrometer (Perkin Elmer, Shanghai, China). The secondary distilled water used in the experiment was purified by the SZ-97 Automatic Pure water distiller (Shanghai Yarong Biochemical Instruments Co. Ltd., Shanghai, China). The thickness of PDEA films was estimated with a Stereo Discovery V12 stereomicroscope equipped with an AxioCam digital camera (Zeiss) (Tongzhoutongde (Beijing) instrument and meter co. LTD, China). The surface scanning electron microscopy (SEM) of the films was obtained with a JSM 7500F scanning electron microscope (JEOL, Tokyo, Japan) at an acceleration voltage of 10.0 kV. After being treated with different pHs, temperature, Na2 SO4 and methanol, the GC disks were immediately placed into liquid nitrogen to freeze the composite film structure. Afterward, they were placed in an ALPHA 1-2 lyophilizer (Beijing Bomaihang Equipment Co. Ltd., Beijing, China) for 24 h to remove all of the water from the samples. Before SEM imaging, the sample surface was coated with a thin gold film with an SBC-12 sputtering coater (KYKY Technology Development Ltd., Beijing, China). The surface morphology of c-MWCNTs was investigated via transmission electron microscopy (TEM, H-7650 Hitachi, Tokyo, Japan). 3. Results 3.1. Characterization of Poly(N,N-Diethylacrylamide) (PDEA) Films Poly(N,N-diethylacrylamide) (PDEA) hydrogel films were polymerized on a GC electrode by radical cross-linking mainly according to previous reports in the literature [25,28] with certain modifications (as described in Section 2.2). A comparison with the commercial pure DEA sample by IR spectroscopy confirmed the formation of PDEA (Figure S1). The characteristic C=C stretching band at 1610 cm−1 observed for DEA monomers was not detected for PDEA, indicating that the DEA monomers were polymerized to PDEA, which was similar to the literature findings [28]. Matrine, an electroactive drug, was used to characterize PDEA film electrodes through cyclic voltammetry (CV) experiments. An obvious oxidation peak at around 1.02 V, which was the same as the literature [29], was observed at the bare GC electrodes due to electrochemical oxidation of matrine (Figure 1A, curve a). For PDEA film electrodes, the electrochemical oxidation peak current (Ipa ) of matrine slightly decreased, and the peak potential shifted positively because the formation of the films hindered the drug from reaching the electrode surface to a certain degree (Figure 1A, curve b), indicating that PDEA hydrogel films had been successfully constructed on the surface of the GC electrode. Sophoridine is an isomer of matrine [30], since they have similar chemical structures (Figure S2). As shown in Figure 1B, sophoridine showed a similar electrochemical behavior to matrine.
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The electrochemical oxidation peak of sophoridine appeared at about 0.91 V, implying that sophoridine implying sophoridine could alsodrug be used as an to electroactive moleculeproperties to investigate the could alsothat be used as an electroactive molecule investigate drug the switching of films. switching properties of matrine films. Inand thesophoridine following studies, matrine andmolecule sophoridine were usedthe as In the following studies, were used as typical probes to study typical molecule probes to study the multiple stimuli-responsive binary films. multiple stimuli-responsive binary films.
Figure 1. Cyclic voltammetry responses (CVs) of 0.010 M (A) matrine and (B) sophoridine at 0.05 V ◦ C in NaCl supporting electrolyte solutions −11and s− and25 25°C in NaCl supporting electrolyte solutions at (a) bare glassy glassy carbon carbon (GC) (GC) electrode electrodeand and(b) (b) poly(N,N-diethylacrylamide (PDEA) film film electrodes. electrodes.
3.2. pH Responsive Behaviors of Poly(N,N-Diethylacrylamide) Films 3.2. pH Responsive Behaviors of Poly(N,N-Diethylacrylamide) Films The pH-sensitive property of PDEA film electrodes was demonstrated by the CV responses The pH-sensitive property of PDEA film electrodes was demonstrated by the CV responses of of matrine and sophoridine. From pH 9.0 to 4.0 at 25 ◦ C, the Ipa of matrine and sophoridine matrine and sophoridine. From pH 9.0 to 4.0 at 25 °C, the Ipa of matrine and sophoridine decreased decreased drastically with decrease of pH (Figure 2A,B). If the Ipa in NaCl solutions at pH 9.0 was drastically with decrease of pH (Figure 2A,B). If the Ipa in NaCl solutions at pH 9.0 was set as the set as the “on” state, whereas that at pH 4.0 as the “off” state, the CV response could be switched “on” state, whereas that at pH 4.0 as the “off” state, the CV response could be switched between the between the “on” and “off” state by switching the film electrode in the solutions between 9.0 and “on” and “off” state by switching the film electrode in the solutions between 9.0 and 4.0. This 4.0. This pH-dependent switchable CV behavior of the system could be repeated several times with pH-dependent switchable CV behavior of the system could be repeated several times with good good reversibility (Figure 2C,D). The response the on-off transitions greatly influenced by reversibility (Figure 2C,D). The response time time of theofon-off transitions was was greatly influenced by the the thickness of the PDEA hydrogel films. The average thicknessofofthe thefilms filmsin in water water estimated estimated by thickness of the PDEA hydrogel films. The average thickness by ◦ C. The response time increased with the thickness stereomicroscopy was 410 ± 30 µm at pH 9.0 and 25 stereomicroscopy was 410 ± 30 μm at pH 9.0 and 25 °C. The response time increased with the of PDEA hydrogel (data not shown), that the diffusion of the molecules through thickness of PDEAfilms hydrogel films (dataimplying not shown), implying that thedrug diffusion of the drug the films tothrough theelectrode surface for electron surface exchange difficult whenmore the hydrogel molecules the films to theelectrode forbecame electronmore exchange became difficult films thicker. In become addition, oxidation peak currents of the drug molecules a linear when become the hydrogel films thicker. In addition, oxidation peak currents of theshowed drug molecules –1 ◦ relationship withrelationship the square root scan rates root fromof0.05 to rates 1.0 Vfrom s at0.05 pH 9.0 andV25 C, pH suggesting showed a linear withofthe square scan to 1.0 s–1 at 9.0 and diffusion-controlled behavior of the drug molecules. 25 °C, suggesting diffusion-controlled behavior of the drug molecules. The of structure and the of PDEA PDEA films films at The probable probable changes changes of structure and the average average thickness thickness of at different different pHs pHs of of solution were investigated by SEM (Figure S3) and stereomicroscopy (Table 1). The results showed that solution were investigated by SEM (Figure S3) and stereomicroscopy (Table 1). The results showed PDEA filmsfilms have have a similar network structure and surface morphology at pH 4.0 and4.0 9.0 and (Figure S3A,B). that PDEA a similar network structure and surface morphology at pH 9.0 (Figure The PDEA component carries no charge regardless of the surrounding pH. In addition, the average S3A,B). The PDEA component carries no charge regardless of the surrounding pH. In addition, the thickness of the films 9.0 (410 µm) stereomicroscopy was nearly the same as that average thickness of at thepH films at pH 9.0estimated (410 μm)by estimated by stereomicroscopy was nearly the at pH 4.0 (405 µm) (Table 1). In the control experiments, matrine and sophoridine demonstrated same as that at pH 4.0 (405 μm) (Table 1). In the control experiments, matrine and sophoridine pH-sensitive properties atCV theproperties bare GC electrode. When pH increased to 9.0, the Ipa demonstratedCV pH-sensitive at the bare GCthe electrode. Whenfrom the pH pH4.0 increased from of matrine and sophoridine increased correspondingly (Figure S4A,B). Thus, the main mechanism was pH 4.0 to 9.0, the Ipa of matrine and sophoridine increased correspondingly (Figure S4A,B). Thus, attributed to the property of matrinetoand which are drugswhich with pK of a value the main mechanism was attributed thesophoridine, property of matrine andalkaloid sophoridine, are alkaloid 1 about 8.20 [31,32]. Their structure contains a tertiary amine nitrogen atom ( N), and an amide drugs 7.80 with−pK a value of about 7.80−8.20 [31,32]. Their structure contains a tertiary amine nitrogen 2 N) which has alkalinity. Amides cannot hydrolyze easily, so its main basicity is derived structure ( atom (1N), and an amide structure (2N) which has alkalinity. Amides cannot hydrolyze easily, so its 1 N. The nitrogen atom in solution can provide lone pair electrons, and so the solutions become from main basicity is derived from 1N. The nitrogen atom in solution can provide lone pair electrons, and alkaline. When pH decreased 4.0, matrine presenttoin4.0, thematrine form ofwas a nitrogen radical, so the solutionsthe become alkaline. to When the pH was decreased presentcationic in the form of which cannot transfer electrons to the electrode. As the amount of matrine in molecular form decreased, a nitrogen cationic radical, which cannot transfer electrons to the electrode. As the amount of the oxidation peaks alsoform decreased or eventhe disappeared. When the increasedor to 9.0, existed matrine in molecular decreased, oxidation peaks alsopH decreased evenmatrine disappeared. as a moleculer form, and the I increased. In addition, the effect of the solution pH on the peak pa When the pH increased to 9.0, matrine existed as a moleculer form, and the Ipa increased. In potentials was investigated (Figure S4). The oxidation peak potentials E hardly shift with increasing pa addition, the effect of the solution pH on the peak potentials was investigated (Figure S4). The
oxidation peak potentials Epa hardly shift with increasing pH 4.0–9.0, which means that the voltammetric behaviors of matrine and sophoridine are not a proton transfer process, just an
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pH 4.0–9.0, which means that the voltammetric behaviors of matrine and sophoridine are not a proton electron process, transferjust process between the process drugs between and the the GCdrugs electrode the experimental transfer an electron transfer and theunder GC electrode under the conditions. experimental conditions.
Figure 2. 2. CVs CVs of of 0.010 0.010 M M (A) (A) matrine matrine and and (B) (B) sophoridine sophoridine at at 0.05 0.05 VVss–1–1 and and 25 25 ◦°C PDEA films films in in Figure C for PDEA pa of 0.010 M (C) NaCl solutions solutions at pH (a) (a) 4.0, 4.0, (b) (b) 5.0, 5.0, (c) (c) 6.0, 6.0, (d) (d) 7.0, 7.0, (e) (e) 8.0, 8.0, and and (f) (f) 9.0; 9.0; Variations Variations of of IIpa NaCl matrine and and(D) (D)sophoridine sophoridine at pH 9.0 and 4.0.The (E)oxidation The oxidation peak current (Ipa) of M matrine at pH 9.0 and 4.0. (E) peak current (Ipa ) of 0.010 M 0.010 matrine in NaCl solutions containingcontaining esterase and ureaseand withurease pH 6.5with (curve after subsequent min reaction matrine in NaCl solutions esterase pHb)6.5 (curve b) after30 subsequent 30 on adding 6 mM (curve a) and after 10 min on10 adding 10 mM ethyl butyrate min reaction on urea adding 6 mM urea (curve a) reaction and after min reaction on adding 10(EB) mM(curve ethyl c). (F) Dependence of c). Ipa(F) onDependence the alternate of addition of EB and urea solutions for and the same PDEA films. butyrate (EB) (curve Ipa on the alternate addition of EB urea solutions for the same PDEA films. Table 1. Average thickness of poly(N,N-diethylacrylamide (PDEA) films estimated by stereomicroscopy under different conditions. Table 1. Average thickness of poly(N,N-diethylacrylamide (PDEA) films estimated by stereomicroscopy underMeasurement different conditions. Conditions Average Thickness/µm ◦
pH 9.0 and 25 C 410Thickness/µm ± 30 Measurement Conditions Average pH 4.0 and 25 ◦ C 405 ± 8 pH 410±±1530 ◦C pH9.0 9.0and and25 40 °C 276 4.0containing and 25 °C 405 ±8 pH 9.0 and pH 25 ◦ C 0.35 M Na2 SO4 207 ± 20 pH 9.0 andpH 25 ◦9.0 C containing 328 and 40 °C20% methanol 276±±1815 pH 9.0 and 25 °C containing 0.35 M Na2SO4 207 ± 20 pH 9.0 andtransfer 25 °C containing methanol ± 18 To verify the electron mechanism20% between matrine and the328 electrodes, the electrochemical behavior of oxymatrine was further studied. The structures of oxymatrine and matrine are similar To verify the electron transfer mechanism between matrine and the electrodes, the electrochemical behavior of oxymatrine was further studied. The structures of oxymatrine and matrine are similar (Figure 3). No oxidation peak exists in the same potential window on the bare
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(Figure 3). No oxidation peak exists in the same potential window on the bare electrode in NaCl electrode at in pH NaCl solutions at pH pH 9.0 (Figure (Figure S5), indicating that the themechanism electron transfer transfer mechanism of solutions 9.0solutions (Figure S5), indicating thatS5), theindicating electron transfer of matrine shown in electrode in NaCl at 9.0 that electron mechanism of matrine shown in Figure 3 is correct. Figure is correct. matrine3 shown in Figure 3 is correct.
Figure 3. Oxidation mechanism of matrine (MT) on the GC electrode. Figure 3. Oxidation mechanism of matrine (MT) on the GC electrode.
The pH pH of of the the solution solution can can be be controlled controlled by by in in situ situ enzymatic enzymatic catalysis catalysis [33–35]. [33–35]. The The hydrolysis hydrolysis The The pH of the solution can be controlled by in situ enzymatic catalysis [33–35]. The hydrolysis reaction of ethyl butyrate and urea was catalyzed by esterase (Est) and urease (Ur). For example, reaction For example, example, reaction of of ethyl ethyl butyrate butyrate and and urea urea was was catalyzed catalyzed by by esterase esterase (Est) (Est) and and urease urease (Ur). (Ur). For the film film was was immersed immersed into into the the pH pH 6.5 6.5 NaCl NaCl solution solution containing containing esterase, esterase, urease, urease, and and matrine matrine the the film was immersed into the pH 6.5 NaCl solution containing esterase, urease, and matrine (Figure 2E, curve b). After the addition of ethyl butyrate, the pH of the solution began to decrease (Figure After the the addition addition of of ethyl ethyl butyrate, the pH pH of (Figure 2E, 2E, curve curve b). b). After butyrate, the of the the solution solution began began to to decrease decrease because of the production of butyric acid, and eventually reached 4.0. CV testing showed small because of the production of butyric acid, and eventually reached 4.0. CV testing showed because of the production of butyric acid, and eventually reached 4.0. CV testing showed aaa small small electrocatalyticoxidation oxidationpeak peak (Figure 2E, curve curve c). Then Thenwas urea was and added, and the the ammonia ammonia electrocatalytic (Figure 2E, 2E, curve c). Then urea added, the ammonia produced electrocatalytic oxidation peak (Figure c). urea was added, and produced under urease catalysis caused an increase of pH. CV scanning was performed again, and under urease catalysis an increase of increase pH. CV scanning performed and the Ipa was produced under ureasecaused catalysis caused an of pH. CVwas scanning was again, performed again, and the IIpa pa was markedly enhanced (Figure 2E, curve a). a). By adding adding ethyl ethyl butyrate and urea urea alternately markedly (Figure 2E, curve a). 2E, By adding ethyl butyrate and urea alternately to the enzyme the wasenhanced markedly enhanced (Figure curve By butyrate and alternately to the enzyme solution of esterase and urease, the switchable behavior could be repeated many solution of esterase and urease, the switchable could bebehavior repeatedcould manybe times between 4.0 to the enzyme solution of esterase and urease,behavior the switchable repeated many times between 4.0 and 9.0 (Figure 2F). and 9.0 (Figure 2F). times between 4.0 and 9.0 (Figure 2F). 3.3. Temperature-Responsive Temperature-ResponsiveBehaviors Behaviorsof ofPoly(N,N-Diethylacrylamide) Poly(N,N-Diethylacrylamide)Films Films 3.3. Temperature-Responsive 3.3. Behaviors of Poly(N,N-Diethylacrylamide) Films The CV CV responses responses of of matrine matrine and and sophoridine sophoridine at the PDEA-film PDEA-film electrode electrode were were extremely extremely The at the The CV responses of matrine and sophoridine the PDEA-film electrode were extremely sensitive phase temperature (LCST) of about about sensitive to ambient temperature, with a lower critical phase transition temperature (LCST) sensitive to ambient temperature, with a lower critical phase transition temperature (LCST) of of about ◦°C 32 (Figure 4A,B), which was similar to that found in the literature [25,36]. When the solution 32 C (Figure 4A,B), which was similar to that found in the literature [25,36]. When the solution 32 °C (Figure 4A,B), which was similar to that found in the literature [25,36]. When the solution temperature was below 32 32 ◦°C, °C, the IIIpa pa values of matrine matrine and and sophoridine sophoridine on on PDEA-film PDEA-film electrode electrode temperature was below C, the temperature below 32 the pa values of matrine and sophoridine on PDEA-film electrode were extremely large in NaCl solutions at pH 9.0, but the CV peak current considerably decreased were extremely large in NaCl solutions at pH 9.0, but the CV peak current considerably decreased were extremely large in NaCl solutions at pH 9.0, but the CV peak current considerably decreased ◦ C °C ◦40 when the thetemperature temperatureexceeded exceeded3232 32◦ C. °C.The The temperatures at 25 25 and °C were selected to study study when temperatures at at 25 and 40 40 C °C were selected to study the when the temperature exceeded °C. The temperatures °C and were selected to ◦ the thermal-sensitive thermal-sensitive switching behavior of thesystem: film system: system: the film was at the the “on” stateCat at 25the °C thermal-sensitive switching behavior of theof film the film was at was the “on” state at state 25 and the switching behavior the film the film at “on” 25 °C ◦ C. The and the “off” state at 40 °C. The temperature sensitive properties of PDEA hydrogel could be “off” state at 40 temperature sensitive properties of PDEA hydrogel could be repeated many and the “off” state at 40 °C. The temperature sensitive properties of PDEA hydrogel could be repeated many times, and and the response time was about 3–6In min (Figure 4C,D). In In the thethe control times, andmany the response timethe wasresponse about 3–6time minwas (Figure 4C,D). the (Figure control experiments, peak repeated times, about 3–6 min 4C,D). control experiments, the peak currents of matrine and sophoridine at the bare electrode showed no obvious currents of matrine and sophoridine at the bare electrode showed no obvious change (Figure S6), experiments, the peak currents of matrine and sophoridine at the bare electrode showed no obvious change (Figure S6), and thus confirmed that the temperature sensitive property of this system and thus confirmed that the temperature sensitive property of this system mainly depends on the change (Figure S6), and thus confirmed that the temperature sensitive property of this system mainlyhydrogel dependsfilms. on the the PDEA PDEA hydrogel hydrogel films. films. PDEA mainly depends on
Figure 4. Cont.
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Figure 4. CVs of 0.010 M (A) matrine and (B) sophoridine for PDEA films at 0.05 V s–1 in NaCl Figure 4.with CVsof of0.010 0.010 matrine and (B)different sophoridine for PDEA at 0.05 s–1(c) in NaCl –1 in solution4. pH 9.0 containing no Na temperatures: 25 27V °C, 29 °C, Figure CVs MM (A)(A) matrine and2SO (B)4 at sophoridine for PDEA films(a) atfilms 0.05°C, V s(b) NaCl solution solution with pH 9.0 (f) containing noat Na 2SO(h) 4 at 39 different temperatures: (a) 25 °C, (b) (c) M 29 °C, ◦Variations ◦ C, ◦°C, ◦ C, (d) 31pH °C, 33 °C, 35Na °C, (g) 37 °C, °C, and (i) 40 °C. of I pa27 of 0.010 (C) with 9.0(e) containing no SO different temperatures: (a) 25 C, (b) 27 (c) 29 C, (d) 31 2 4 –1 in (d) 31 °C, (e) 33 °C, (f) 35 °C, (g) 37 °C, (h) 39 °C, and (i) 40 °C. Variations of I pa 9.0 of 0.010 M (C) ◦ ◦ ◦ ◦ ◦ NaCl solutions with pH at 25 °C and matrine and (D) sophoridine for PDEA films at 0.05 V s (e) 33 C, (f) 35 C, (g) 37 C, (h) 39 C, and (i) 40 C. Variations of Ipa of 0.010 M (C) matrine and (D) ◦ C and NaCl solutions pH40 9.0◦ C. at 25 °C and matrine (D)PDEA sophoridine PDEA at 0.05 V s–1 inwith 40 °C. and for sophoridine films atfor 0.05 V s–1films in NaCl solutions pH 9.0 at 25with 40 °C.
Scanning electron microscopy (SEM) also supported the structural changes of PDEA films with ScanningAfter electron microscopy (SEM) also supported the oftemperatures, PDEA films with temperature. being immersed inin NaCl solutions with pHstructural 9.0 at temperatures, the films temperature. After being immersed NaCl solutions with pH 9.0different atchanges different the ◦ C temperatures, temperature. After being immersed in NaCl solutions with pH 9.0 at different the showed a network structure with numerous microsized pores and channels at 25 (Figure 5, panel films showed a network structure with numerous microsized pores and channels at 25 °C (Figure a), 5, films a network structure with numerous poresatand channels atstructure 25panel °C (Figure 5, whereas displayed compact and smooth surface at 40 ◦surface C (Figure 5,40 panel c). The change panelshowed a),they whereas theyadisplayed a compact and microsized smooth °C (Figure 5, c). The panel a), whereas theyfurther displayed a compact smooth surface 40 °C (Figure panelinc). The with temperature was verified by further the and change of films with temperature water. structure change with temperature was verified by thickness the at change of films 5,thickness with ◦the structure change with temperature was further verified by change of films thickness with The average film thickness estimated by stereomicroscopy at 40 C became 276 µm, much smaller than temperature in water. The average film thickness estimated by stereomicroscopy at 40 °C became ◦ C (410 temperature insmaller water. The average thickness estimated that at 25much µm) (Table 1). at 25film 276 μm, than that °C (410 μm) (Table 1). by stereomicroscopy at 40 °C became 276 μm, much smaller than that at 25 °C (410 μm) (Table 1).
(a) (a)
(b) (b)
(c) (c)
(d) (d)
Figure 5. Top-view scanning scanning electron electron micrographs micrographs (SEM) (SEM) of of PDEA PDEA hydrogel hydrogel films on the GC 5. Top-view ◦ C, Figure 5. surface Top-view scanning electron (SEM) of no PDEA hydrogel films onatthe electrode in NaCl solutions withmicrographs pH 9.0 Na and methanol methanol 25 GC °C, 9.0 (a) (a) containing containing no Na22SO 44 and ◦ SO and methanol at 25 °C, electrode surface inMM NaCl with pH 9.0at no Na2no (b) containing 0.35 Na methanol 25 C, (c) at 2solutions SO44but butno no methanol at(a) 25containing °C, (c)containing containing no4Na Na SO containing 0.35 Na 2 SO 22SO 44 and methanol 2SO4methanol but no methanol (b)◦°C, containing 0.35 M Na20% 40 C, and (d) containing at 25 ◦°C. C. at 25 °C, (c) containing no Na2SO4 and methanol at 40 °C, and (d) containing 20% methanol at 25 °C.
Sensors 2018, 18, 3358 Sensors 2018, 18, x FOR PEER REVIEW
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3.4. Na22SO SO44-Responsive -Responsive Behaviors Behaviors of of Poly(N,N-Diethylacrylamide) Poly(N,N-Diethylacrylamide) Films Films The CV at at PDEA-film electrodes were alsoalso sensitive to the CV responses responsesofofmatrine matrineand andsophoridine sophoridine PDEA-film electrodes were sensitive to ◦ C, matrine and salt concentration in solution. When the testing solution contained no Na SO at 25 the salt concentration in solution. When the testing solution contained no Na 2 SO 4 at 25 °C, matrine 2 4 sophoridine showed relatively large oxidation peaks, and theand filmsthe were at the “on” When the and sophoridine showed relatively large oxidation peaks, films were at state. the “on” state. solution contained 0.35contained M Na2 SO40.35 under matrine and sophoridine oxidation When the solution M the Nasame 2SO4 conditions, under the the same conditions, the matrine and peaks were undetectable, and the were at theand “off”the state. Na2at SO4the -sensitive switching sophoridine oxidation peaks werefilms undetectable, filmsThe were “off” state. The behaviors of matrine and sophoridine also reversible. The “on”were and also “off”reversible. states of the system Na2SO4-sensitive switching behaviors were of matrine and sophoridine The “on” could be repeated several times (Figure For theseveral controltimes experiments, peakFor currents of the and “off” states of the system could 6C,D). be repeated (Figure the 6C,D). the control drug probes at the barecurrents electrode distinct change solutions containing 0 and change 0.35 M experiments, the peak of showed the drugno probes at the bare in electrode showed no distinct Na S7). That is, 0.35 the Na SO2SO of Na matrine and sophoridine also in solutions containing 0 and M 2Na 4 (FigureCV S7).behaviors That is, the 2SO4-sensitive CV behaviors 2 SO4 (Figure 4 -sensitive should be ascribed to the PDEA in the films. of matrine and sophoridine also component should be ascribed to the PDEA component in the films.
–11 and Figure 6. C in 6. CVs of of 0.010 0.010 M M (A) (A) matrine matrine and and (B) (B) sophoridine sophoridine for forPDEA PDEAfilms filmsat at0.05 0.05VVss–11 and 25 25 ◦°C NaCl solutions with pH 9.0 containing (a) 0 M, (b) 0.05 M, (c) 0.10 M, (d) 0.15 M, (e) 0.20 M, (f) 0.25 M, solutions with pH 9.0 containing (a) 0 M, (b) 0.05 M, (c) 0.10 M, (d) 0.15 M, (e) 0.20 M, (f) 0.25 (g) 0.30 M and (h) 0.35 M Na SO . Variations of I of (C) 0.010 M matrine and (D) 0.010 M sophoridine pa of Ipa of (C) 0.010 M matrine and (D) 0.010 M M, (g) 0.30 M and (h) 0.352 M 4Na2SO4. Variations for PDEA films 0.05 films V s–1 at and 25 V◦ Cs–1inand NaCl with pH 9.0 with containing M and 0.350 M sophoridine for at PDEA 0.05 25 solutions °C in NaCl solutions pH 9.00containing M of Na0.35 2 SOM 4 . of Na2SO4. and
Other and NaCl, Other types types of of salts, salts, such such as as MgSO MgSO44,, NaNO NaNO33,, and NaCl, can can also also cause cause the the phase phase transition transition of of PDEA film and the CV switchable behavior of drug probes (Figure S8). The main difference PDEA film and the CV switchable behavior of drug probes (Figure S8). The main difference between between 2 − (0.20 M) < Cl− (0.54 M) these salts was was their theirdifferent differentcritical criticalphase phase change concentration 4 these salts change concentration (SO(SO 42− (0.20 M) < Cl− (0.54 M) < NO3− − +
